The present invention relates generally to laser hardened materials and structures, and more particularly to a novel optical filter structure for selectively blocking laser radiation of predetermined wavelength while passing radiation of other wavelengths.
Optical switching devices comprising transition or switching materials which are optically transparent in one state but which transform to an opaque metallic state when heated to a characteristic transition temperature are well developed for applications such as optical filters, modulators, laser output couplers, and the like. These devices are generally characterized by a transition from substantial transparency below the characteristic transition temperature to substantial opacity above that temperature, at all wavelengths.
In accordance with the teachings of copending application Ser. No. 07/172,119, which teachings are incorporated herein by reference, an optical filter structure was described for selectively blocking radiation of predetermined wavelength comprising a layer of material characterized by transition from non-metal to metal, corresponding to a change from transmissive to reflective states upon being heated to a characteristic transition temperature, deposited on a slab of dielectric material in the form of a plurality of multipole elements in an array of predetermined spacing between adjacent rows and columns of the array. The filter of the 07/172,119 application is characterized by substantial transparency to all radiation wavelengths below its characteristic transition temperature, and opacity only to the predetermined wavelength, and substantial transparency to all other wavelengths, above the transition temperature, and is constructed by applying a film of thermal switching material in the form of an array of microwave-type multipole elements of optical dimensions, the film being applied to a dielectric slab, or sandwiched between two dielectric slabs.
For purposes of describing the invention and defining the scope thereof, the term "optical" shall, in accordance with the customary usage, be defined herein to include only ultraviolet, visible, near infrared, mid-infrared and far infrared regions of the electromagnetic spectrum lying between about 0.1 to about 1000 microns (see e.g. Optical Physics, by Max Garbuny, Academic Press, New York, 1965, pp 1-6), and more specifically to the range of from about 0.2 micron, the approximate lower limit of operation of fine quality quartz lenses (Garbuny, p 280), to about 50 microns, the approximate upper limit of operation of long wavelength transmitting material such as KRS-5 (thallium bromide-iodide ionic crystal) (Garbuny, p 282).
The invention provides a substantially improved optical reflection band filter capable of actively or passively sweeping a preselected range of wavelengths and locking in on a wavelength within that range to effectively block that wavelength while substantially passing all others. It is noted that, in addition to the preselected wavelength sought to be blocked, a basic resonant wavelength corresponding to the overall size of the elements may also be blocked. The invention is constructed by depositing a periodic array of microlithographic patterns of conducting metal in the form of microwave antenna type multipole elements of optical dimensions, preselected according to a desired characteristic wavelength range to be swept. The metallic elements comprise small spaced-apart metal segments interconnected by sections of superconducting-to-insulator phase transition materials deposited in the spaces between adjacent metallic segments. Each successive phase transition section from the center of each pattern outwardly has predetermined lesser thickness than the next inwardly situated transition section. Upon heating the array by a controllable heater contacting the filter or by impinging laser radiation, the interconnection sections switch from superconducting to insulating at different temperatures or at different heating rates for each set of interconnections, providing wavelength sweeping over the characteristic preselected wavelength reflection band. The invention is therefore a substantial improvement over the totally opaque switching reflection filter, the non-switching metallic element array reflection-band filter, the fixed wavelength switching element reflection band filter, and other wavelength responsive laser hardening devices.
It is therefore a principal object of the invention to provide an improved optical filter.
It is a further object of the invention to provide an optical filter for selectively blocking laser radiation of particular wavelength while passing radiation of other wavelengths.
It is yet a further object of the invention to provide a switchable filter incorporating insulating-to-superconducting materials to provide selective wavelength filtering.
It is yet another object of the invention to provide an optical filter having fast response time.
It is yet a further object of the invention to provide a filter capable of sweeping a reflection band to reflect a preselected wavelength within that band.
These and other objects of the invention will become apparent as the detailed description of representative embodiment proceeds.